Antenna system for vehicles



y 1933 as. FOSTER 2,157,532

ANTENNA SYSTEM FOR VEHICLES Filed May 19, 1936 Z w RAD/0 7 RECEIVER INVENTOR 0. E. F STER) BY 4 ATTORN EY Patented May 9, 1939 UNITED STATES ANTENNA SYSTEM FOR VEHICLES ware Application May 19,

3 Claims.

This invention relates to a loop antenna which Will pick up signals over a desired frequency band and may be tuned from a remote point.

An object of the invention is to provide a band 5. pass filter arranged to pass a broad band of radio frequency currents and in which one of the inductances or coils of the filter is exposed to a high frequency radiation field.

A further object of the invention is to provide a wide band pass filter so arranged that its input coil is exposed to the concentrated electromagnetic field beneath an automobile when the latter is exposed to high frequency electromagnetic or radio waves.

Further objects of the invention will become apparent to those skilled in the art as the description thereof proceeds. The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both "ts organization and method of operation will est be understood by reference to the following description taken. in connection with the accompanying drawing in which I have indicated diagrammatically two arrangements whereby the invention may be carried into effect.

In the drawing:

Fig. 1 shows a schematic circuit diagram of the antenna system as applied to a radio receiver and,

Fig. 2 shows diagrammatically the antenna system as installed on an automobile.

Referring to Fig. 1 the antenna system is shown as a band pass filter comprising an input coil L2, an output coil L2", a series connected coil L1 and two condensers C2 and C2 which are shunted across the coils L2 and L2" respectively. The output coil is coupled to the input terminals of a radio receiver I, the coupling shown being a 40 transformer in which coil L2" is the primary and which is inductively coupled to the secondary L3. As shown, the terminals of the secondary are connected to the grid and cathode of the first radio frequency amplifier stage 2 of the receiver, a variable condenser C3 being provided for the purpose of tuning its circuit to the desired signal frequency. It will be understood that the receiver may be of the tuned radio frequency type having several coupled stages each tuned to the signal frequency or it may be of the superheterodyne type.

I propose to use a loop antenna as the input coil L2 of the filter and to mount this coil in a concentrated electromagnetic field such as exists, for example, between the body of a metal 1936, Serial No. 80,489

automobile and ground. Under such conditions and in the efiective field of a broadcasting station transmitting radio waves on a carrier frequency, the antenna system will act as a band pass filter and transmit the energy picked up to the input terminals of a radio receiver mounted at any desired place in the car.

As an example of such installation, assume a case where the capacity of the loop antenna and its connecting leads to coil L1 constitutes the capacity C2 of Fig. 1 and equals micro-microfarads. Also if the filter is to pass currents of all frequencies in the present broadcast band, the upper cut-off frequency f2=l500 kilocycles and the lower cut-off frequency f1=600 kilocycles. 15 The remaining constants of the filter may then be calculated from the known formulae given on page 316 of Transmission Networks and Wave Filters by T. E. Shea (1929 edition).

25 1 (3) 1r(f f )R Applying these formulae we have,

2600 ohms 3O 520 microhenries The characteristic impedance (R) of the filter is therefore equal to 2660 ohms and since L2 is the total inductance of the shunt section of the filter and is composed of inductances L2 and L2" in parallel, each of the latter will have a value of 1040 microhenries. When connected to the tuned input circuit of the receiver the mutual inductance M is preferably adjusted until the reflected impedance of the tuned circuit most nearly matches the value of R over the desired range of frequencies to be transmitted.

As shown in Fig. 2 the filter input coil L2 con sists of a loop antenna 3 composed of several complete turns, the loop being mounted under the metal body of an automobile 4 between the floorboards of the car and the ground 5 in which space there is a concentrated electrical field when the car body is picking up radio waves emitted =920 microhenries from one or more broadcasting stations. As shown, the plane of the loop is arranged at right angles to the ground. As regards the operation of the device the voltage pickup of a loop antenna is afiected by nearby metallic bodies and in some cases not only is the voltage pickup increased but the electro-magnetic field acting upon the loop may be composed of the ordinary field plus a reflected or re-radiated or scattered field from the metallic body which latter field may be out of time phase with the direct field so as to produce a resultant rotating field which induces voltage in the loop regardless of its orientation about a vertical axis, thus eliminating the usual directions of zero response of a simple loop antenna in a simple non-rotating field. A pair of leads 6, I connect the ends of the loop to the remaining elements of the band pass filter which may be mounted within the receiver I indicated by the dotted lines.

As shown by the theory of filters, the inductance L2 is not limited to the value given above while the frequency range remains the same and itmay vary over quite wide limits if at the same time the values of L1, R and C2 are adjusted correspondingly. For example, L2 may consist of a single turn loop having an inductance of the order of 20 microhenries. Also, while I have shown the loop as being mounted under the car in the electrical field between it and the ground it will be understood that my invention is not limited thereby but that the loop may be located in other positions about the car such as above it, or at the rear or front of its main body portion. Also the capacity C2 need not be a physical condenser since it may be composed of the capacity of the loop together with the capacity between the connecting leads 8, l of Fig. 2 especially in a case where these leads are fairly long due to the receiver being mounted at a point remote from the loop. Any suitable supporting device (not shown) may be used for supporting the loop in position underneath the car.

In the usual automobile installation, the car frame acts as the return path for the ignition system and in order to prevent high frequency currents generated by the ignition system from getting into the receiver input circuit, I preferably use two leads to connect the loop to this circuit, neither the loop nor either lead being connected to the car frame or chassis. Furthermore, since the radio field has substantially the same strength at all points in each horizontal plane under the car whereas the radiation from the ignition system is more intense at the front of the car, by mounting the loop toward the center or rear of the car, the ignition noise picked up by the antenna system may be reduced without affecting its utility as a collector of radio signals.

Having described my invention, what I claim as new and desire to secure by Letters Patent is:

1. In a radio receiving system adapted for use in a vehicle containing a source of spark interference, the combination of a loop antenna located in the vicinity of a metallic portion of said vehicle and remote from the field of sail source and a band pass filter including shunt inductance elements connected between said loop and the remaining portion of said receiving system, said loop providing the input shunt inductance of said filter as Well as acting as a pickup device for signals.

2. An arrangement as described in the next preceding claim characterized by that a resonant circuit is coupled to the output of said filter, resonant circuit being arranged to reflect into the output of said filter a resistance approximately equal to the characteristic impedance of said filter at at least one portion of the band of frequencies to be received.

3. In combination with a vehicle having a metallic body portion, a radio device mounted in said vehicle, a loop antenna located in the vicinity of a metallic portion of said body, a band pass filter having one end coupled to the input termi-- nals of said radio device and its other end coupled to said loop antenna, said band pass filter including shunt inductance elements connected between said loop and the radio device, said loop providing the input shunt inductance of said filter as well as acting as a pickup device for signals.

DUDLEY E. FOSTER. 

